I posted this guy here a while ago. His name is Aspen and he's a pit/lab mix. I work as a caretaker for a mobility disabled adult. Last year she adopted Aspen as a new service dog to replace her 7 year old Beagle/Pitt mix who is set to retire very soon. The ideal age for a new service dog is about 9 months to a year, which makes it a tough search especially with larger breeds. 
My client needs a fairly muscular dog for help with bracing, and a bigger dog can provide some fall protection without getting hurt himself. 
Aspen was a year and a half old and had spent almost his entire life in shelters. He was picked up as a puppy and he had missed a lot of socialization and was a bit developmentally lagging. 
When we brought him home, he was about 70 lbs, had a troubled stomach and was a bit mouthy and hyper. He was also very sweet and people-positive with no signs of aggression. Just a little rough around the edges.

Happy to report after a year of training and stable living, he is showing huge improvements. He is now a beefy 95 lbs. He's very strong but is much more gentle.
He is rarely mouthy, and avoids jumping up on people. He is still a whirlwind when he gets excited, but he's learned to calm himself, eventually.
He's able to accompany me on some errands for mom, and he is getting more patient and relaxed every day. 
He's a very good cuddler, still a bit needy as pittie babies are known to be. He is very attached to me and mom, but he's much more comfortable about me leaving after my shift ends. 

Anyway, as a caretaker, I think pitbull breeds can be fantastic service dogs. They're smart, emotionally intuitive and strong. You can hone their attachment into a useful skill, and their attentive, curious nature makes them very quick to learn.

 Aspen was a tough case, but we both believed he had potential and even with his background, he's come so far. There's still work to do before he's fully ready to get certified. But, we think his predecessor should be looking at a nice retirement within the next year. He's earned it. 

My Care Client's Service Dog in training (update!)